Off Topic: DIY Electric Bike Build

Every once in a while I get all nutty processor and build something in the garage 🙂 This time I built my own Electric bike!

Started by getting a free mountain bike which was being thrown out.

I had a BMX bike in the bits pile so took off the main sprocket and ground out the center hole. I got it to fit snugly onto the rear wheel. I’ve temporally held it in place with some epoxy and bolts.

Here you can see the sprocket mounted on the rear wheel.

My thinking here is that I would mount a rear saddle bag rack and mount a motor on the rack. Then I could run the chain from the BMX doner bike around the motor and rear wheel sprocket.

I had a small motor controller laying around which I wanted to use. I had no idea what I was doing but lets just say this wasn’t nearly powerful enough. I’m just including the failures for fun.

Here I salvaged the freewheel from the MBX bike and welded a washer to one side so that it could be mounted to the motor. My thinking here is that the motor will turn the rear wheel when powered. When the bike is rolling under its own power the motor wont turn because the freewheel will just spin and not the motor.

EEkk that’s horrible welding but nothing a grinder wont fix. FYI Washers are normally coated so they don’t weld well unless you grind off the coating.

Ok so I sourced a car starter motor and modified it so I could attach the freewheel.


Here you can see the chain running through it and how it would connect to the rear wheel sprocket.

Needed somewhere to mount the motor so build a box for it to sit inside.

Here the saddle bag rack, motor box are all fitted and connected up to the rear wheel.

She’s starting to get there! but you remember I said the small speed controller didnt work well.. yes it kinda exploded so I upgraded to a motor controller from an old golf cart. Here is a link to an old ebay listing:

I was really surprised how easy it was to get working. Just connect a couple of 12v motor bike batteries together to make 24v. Connect that to the controller power in. Connect an on/off switch a patentiometer to control speed and connect the motor to the correct terminals and bingo!

A little bit messy but this was fine for testing.

I temporally mounted the patentiometer and on/off switch to the handbars and went for a test ride!

OK soooo the car starter motor didnt go so well! 🙁 It seemed to have enough power but as soon as I got my fat ass on it there was virtually no power assist. There was a little but nothing worth all this effort.

I went out and found a proper 500w 24v DC motor and hooked it up instead. Just FYI if you try this yourself go for the 1000w version for super speed!

Hooked up the freewheel and tested the motor with the controller.

Rather than rebuilding the box I decided to just use the mounting bar and directly attach the motor to that. It worked really well!

The motor fits snugly under the saddle bag rack and above the rear wheel.

This new setup worked a treat! It easily gets me to work and back on a single charge. Round trip its about 22 miles and will go under its own power at around 12mph. Not really fast but its enough to get me to work without needing a shower and still lets me pedal and get some of the health benefits of riding.

I gave the whole bike a few coats of mat black and I love it.

Happy building!

Author: Ian@SlashAdmin

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  1. Hi,
    This is a quite interesting article. How did you end up permanently fixing the sprocket onto the bicycle wheel, and how did you initially grind the center hole of the sprocket? I’m working on a similar project, and have similar issues to yours. Looking forward to hearing from you.


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  2. Hi Sammy! Actually I ended up using some body repair fibreglass filler.. no joke! It’s been rock solid for last couple of years 🙂 spread it on the back and front of the sprocket and get some around the spokes and it locks it in place.

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